The disc centrifuge photosedimentometer has proved to be an excellent instrument for providing particle size and particle size distribution information to pigment and latex manufacturers and formulators. In use, a disc centrifuge photosedimentometer operates by forcing particles (generally of less than about 2 microns in size) under high centrifugal force radially outwardly through a spin fluid or medium. Here the particles segregate into sizes, the larger traversing the medium more quickly, and the smaller taking a longer time. In general, bands of particles of approximately the same size are created in the medium and can be measured optically with the aid of light or other suitable radiation to which the particles are opague, which radiation traverses the spin medium in an axial direction. The bands so created intercept the radiation and can be analyzed and the data obtained converted to particle size and/or distribution curves.
A particularly satisfactory method for photosedimentometric analysis is that described and claimed in U.S. Pat. No. 3,475,968 dated 4 Nov. 1969. U.S. Pat. No. 4,311,039 dated 19 Jan. 1982 describes and claims an apparatus which is especially effective in practicing the method of the aforesaid U.S. Pat. No. 3,475,968. The apparatus of U.S. Pat. No. 4,311,039, with or without "cut timer" modifications, can be used most satisfactorily in practicing the present method. Thus, the disclosure of U.S. Pat. No. 4,311,039 is incorporated herein by reference.
U.S. Pat. No. 4,478,073 dated 23 Oct. 1984 provides a new and alternative method for making centrifugal disc photosedimentometric analyses which also allows simplification in the aforesaid improved apparatus of U.S. Pat. No. 4,311,039, as well as the apparatus of U.S. Pat. No. 3,475,968. Power interrupting means utilized in the devices of both the latter patents can, if desired, be eliminated. In any event, such power interrupting means are not used in practicing the present invention or the invention of U.S. Pat. No. 4,478,073. Moreover, the results obtained according to the new method as will be shown herein, are improved over the results obtained with U.S. Pat. No. 4,478,073 or U.S. Pat. No. 4,311,039 using the buffered line start method described in U.S. Pat. No. 3,475,968 and in the prior art mentioned below. Especially, the improved method of the present invention contemplates the formation of the spin fluid including a density gradient, internally of the disc but without the necessity for interrupting the spinning of the disc in order to generate a necessary density gradient.
Respecting spin fluid media, the homogenous method, the line start, and the buffered line start methods, are all well known. The latter two systems involve preparations of spin fluids or spin media in situ, i.e., during high speed rotation. Reference may be had to Chemical Analysis, "Direct Characterization of Fine Particles" by B. H. Kaye, Volume 61, John Wiley & Sons (1981) Pages 189-226; Schnelle DichtegradientenZentrifugation dispergierter Teilchen, Lange, Colloid & Polymer Sci., 258, p. 1077-1085 (1980) CODEN CPMSS); "Zur Sedimentationsanalyse wassriger Kunkstoffdispersionen mit der Scheibenzentrifuge" Langer, Colloid and Polymer Sci., 257, p. 522-532 (1979) CODEN CPMSE; Zur Genauigkeit der Teilchengrossenanalyse durch Sedimentation im Zentrifugalfeld" Alex, Dissertation for PhD in Engineering 15 Jul 1972 Universitat Kalsruhe. Reference may also be had to the following U.S. Pat. Nos. 2,956,434 Donoghue; 3,237,455 Slater; 3,243,106 Atherton, and the following articles; Jones et al "Particle Size Analysis of Inorganic Pigments" Anal. Chim Acta 38, (1967) 143-146; Proc. Soc. Anal. Chem., Particle Size Analysis Group, July 1966 "The ICI--Joyce Loebl Disc Centrifuge", Jones; Powder Technology, 13 (1976), 215-221 "Particle Size Determination of Pigments with a Disc Centrifuge," Brugger; "Centrifugal Sedimentation, Fraser, Pigments Handbook III-A-d-2-ii, pp. 53-62; "The Accuracy and Precision of the Centrifugal Disc Photosedimentometer Method of Particle Size Analysis", Burt, Powder Technology, 1, (1967) 103-115; "Size Analysis of Organic Pigments using the ICI-Joyce-Loebl Disc Centrifuge", Beresford, Jour. Oil & Color Chem. Assoc. Vol. 50, (1967) 594-614; "The Measurement of Particle Size and its Practical Significance in Vat Dye Quality", Atherton et al, J. Soc. Dyers Colourists 80, p. 521-526 (1964); G. P. Langer, "Sedimentation Analysis of Aqueous Polymer Dispersions with a Disc Centrifuge;" Colloid and Polymer Sci., 257, 522-532 (1979); and Scarlett et al, "The Two Layer Method of Particle Size Analysis", Particle Size Analysis Conference, 1966 pages 242-267.
From the foregoing patents and literature, it will be seen that the industry method of choice is the Joyce-Loebl buffered line start method for particle size determination. Because of its improved structure and control means, the apparatus described and claimed in U.S. Pat. No. 4,311,039 is especially useful in carrying out the buffered line start method.